EPA radon action level 5
https://atomic-wiki.win/index.php/Why_Searching_for_Radon_Testing_Near_Me_Is_a_Smart_First_Step_for_St._Louis_Homeowners
The EPA radon action level is set at 4 picocuries per liter of air, meaning that if your home's test shows radon at or above this concentration, you should take steps to reduce it. This level isn't an absolute safety threshold, because radon exposure carries some risk at any amount, but it's the point where the EPA recommends mitigation to lower lung cancer risks.